In recent years, in a semiconductor industry, as a semiconductor device becomes more integrated, a finer pattern over a transfer limit of related-art photolithography using ultraviolet light becomes necessary. EUV lithography, which is an exposure technology using extreme ultra violet (hereinafter referred to as “EUV”) light, is promising for enabling formation of such a fine pattern. In this case, EUV light means light in a wavelength band of a soft X-ray region or a vacuum ultraviolet radiation region, and specifically, light at a wavelength of about 0.2 nm to about 100 nm. As a mask for transfer used in this EUV lithography, a reflective mask is proposed. Such a reflective mask is a mask in which a multilayer reflective film for reflecting exposure light is formed on a substrate and an absorber film for absorbing exposure light is formed on the multilayer reflective film in a pattern.
The reflective mask is manufactured by forming an absorber film pattern by photolithography or the like from a reflective mask blank including a substrate, a multilayer reflective film formed on the substrate, and an absorber film formed on the multilayer reflective film.
In general, the multilayer reflective film and the absorber layer are formed using a film forming method such as sputtering. In the film formation, the substrate for the reflective mask blank is supported in a film forming apparatus by support means. As the support means for the substrate, an electrostatic chuck is used. Therefore, in order to facilitate fixing of the substrate by the electrostatic chuck, a conductive film (back surface conductive film) is formed on a back surface of an insulating substrate for the reflective mask blank such as a glass substrate.
As an example of the conductive film coated substrate, in Patent Document 1, there is disclosed a conductive film coated substrate for use in manufacturing a reflective mask blank for EUV lithography, which has a feature in that the conductive film contains chromium (Cr) and nitrogen (N), an average concentration of N in the conductive film is 0.1 at % or more and less than 40 at %, a crystal state of at least a surface of the conductive film is amorphous, a surface roughness (rms) of the conductive film is 0.5 nm or less, and the conductive film is a composition gradient film in which the N concentration in the conductive film changes along a thickness direction of the conductive film so that the N concentration on the substrate side is low and the N concentration on the surface side is high.
Further, in Patent Document 2, there is disclosed a conductive film coated substrate for use in manufacturing a reflective mask blank for EUV lithography, the conductive film being formed on the substrate, which has a feature in that the conductive film includes at least two layers of a layer formed on the substrate side (lower layer) and a layer formed on the lower layer (upper layer), the lower layer of the conductive film contains chromium (Cr), oxygen (O), and hydrogen (H), and the upper layer of the conductive film contains chromium (Cr), nitrogen (N), and hydrogen (H). Further, in Patent Document 2, it is disclosed that a surface roughness (rms) of the conductive film is 0.5 nm or less.